Method and means for increasing bending fatigue strength of rotary steel drill pipes



June 9, 1953 J. J. DUNN ETAL 2,641,488

METHOD AND MEANS FOR INCREASING BENDING FATIGUE STRENGTH 0F ROTARY STEEL DRILL PIPES Filed Nov. 12, 1947 Patented June 9, 1953 Mar-Henson MEANS FOR moccasins BENDING-FATIGUE STRENGTH F no... TARY STEEL. DRILL mess.

Jerry Jay Dunn, Ellwood Gity, and" William O. @linedinst, Pittsburgh, Pa.,v assignors to United States: Steel'tlornoration, a corporationiof New Application November 12, 1947'; ScriaLNo. 785,432

3 Claims.

for increasing bending fatigue strength of rotary steel drill pipe.

Rotary dril'l pipe commonly comprises a number o'f stee'l pipe sections joined by interiorly s'crewthreaded couplings that engage exteriorly threaded en'd' regions of the sections. cause offailure in such pipe is through bending fatigue at the threaded joints, since the threads lower the bending fatigue strength of the pipe sections; Where such failure occurs, transverse fatigue cracks appear in the pipe section in the region of the last thread engaged by the coupling. Such cracks are produced by repeated bending ofthe pipe as it rotates within drill holes. Bending stresses are aggravated when the holes are not perfectly straight or when the drill string is partially in compression.

The'stresses that cause fatigue failure in rotating drill pipe are distinctly different from those that cause fatigue failure in relatively fixed screw thr aded fasteners. In the former fatigue failure is caused by repeated bending stresses, while in the latter fatigue failure is caused by repeated variations in longitudinal stresses. In view of this distinction the present application is confined to methods and means for increasing the bending fatigue strength of steel drill pipe.

An object 01 the present invention is to provide improved methods for increasing fatigue strength of steel drill pipe in regions of threaded joints, such methods involving coldworking of the thread roots.

A further object of the invention is toprovide methods for increasing fatigue strength oisteeldrill pipe threads beyond that obtainable through cold working conventional threads, the further increase being obtained by cutting the threads, and particularly the fade-out regions thereof, according to an improved design and subsequently cold working the roots.

A. further object oi the invention is to provide threaded steel drill pipe having improved means ior restoring fatigue strength in the threaded region to approximately that of unthreaded pipe, means including threads having cold worked root portions.

A further object of the invention is to provide externally threaded steel drill pipe having improved means for counteracting the loss in tetigue strength in threaded regions, such means. including threads preferably of the \htype having roots rounded and cold worked at least from within the fade-out region to beyond the last A frequent Z coupling-engaging thread, the sides and areas of the threads not being cold Worked;

Irr accomplishing these and other objects of the-present invention we have provided improved details of structure, preferred forms of which are shown in the accompanying drawing, wherein:

Figure l is a 'l'ongitudinal sectional View through a portion of a drill pipehav-ing threads out and cold worked in accordance with the preferred embodiment of the present invention, and also illustrating the cold working tool engagement with the threads; and

Figure 2' is a longitudinal sectional-*vi'ew through aportion of a drill pipe having threads cold worked in accordance with an alternative em bodiment of the invention.

Referringmore in detaii to the drawing? In Figure 1* there is shown a portion off-a steel drill pipe it having a slightly tapered end. Ex terior screw threads I i are formed in said tapered end for engagement by a coupling, not shown. The threads may-be eithersingleor multiple and are preferably of the rounded \ltype, having rounded roots i2, rounded crests t3 andsides M. The threads have a fade-out region It extending into the body of the pipe section away from the coupling-engaging portion. In the preferred embodiment the thread roots in the fade-out region are also rounded and the fadeout angle, designated X, is as small as possible. consistent with a relatively short fade-out region. Where possible, the fade-out angle is the same as the taper angle.

In accordance with the present invention, roots [2 of the coupling-engaging threads are cold" worked as indicated at it. Such cold working hardens the material at the thread roots and has been found to restore the fatigue strength of the threaded region to approximately that of unthreaded regions.

The roots are cold worked to a diameter-re. duction of at; least 0.0.0.1. lower limit is. fairly critical since the cold. workin mustiirst overcome the m chining imperfections or the:

While there ar certain file. Therefore, we wish to be restricted to an upper limit of cold work of not less than 0.008" diameter reduction, since satisfactory improvement in fatigue strength can be obtained by exceeding the optimum amount to at least this extent, although the operation is more diflicult.

Only the thread roots are cold worked, since cold working makes the material hard and brittle and it is desired that the body of the threads be relatively soft for engagement with coupling threads and to prevent breakage. Since fatigue failure almost invariably occurs in the region of the last thread'on the pipe section engaged by the coupling, theoretically the increased fatigue strength could be obtained by cold working the root of only the last engaged thread. There are, however, practical difiiculties to such practice, since it is not known in wall 18.50 lbs., threaded and coupled drill pipe of API Grade E steel. The pipe ends were upset to an outside diameter of 5.20 inches and an inside diameter of 3.750 inches and threaded with .200 inch pitch-.400 inch lead, I rounded V-threads of API type. The threads in the fadeout region had the rounded roots preferred for cold working. The joint without cold worked roots had a satisfactory endurance limit or fatigue strength at a bending moment that creates a stress of 8,000 pounds per square inch maximum. Companion joints having the pipe thread roots cold worked according to the teachings of this invention resulted in fatigue strength of 19,000 pounds per square inch, representing an improvement of approximately 140%.

. While rounded roots are preferable for accomplishing the maximum gain by cold working, it

advance exactly which thread is to become the last engaged thread and since a continuous smooth thread root is desired. For these reasons. the preferred practice is to cold work the entire thread root, but to increase the amount of cold reduction progressively from the end of the pipe to a maximum within the diameter reduction range stated in the region which contains the last engaged thread, and then to decrease the reduction progressively through the fade-out thread.

In the preferred embodiment, shown in Figure 1, the rounded fade-out region of the thread is also cold worked. The alternative embodiment of the invention, shown in Figure 2, is similar to the embodiment shown in Figure 1, except that the threads in the fade-out region, designated 15a, are cut conventionally, and only the coupling engaging threads are cold worked. The fade out threads lack the rounded roots, and the fade-out angle, designated Y, is somewhat greater than in the preferred embodiment. Cold reduction of the thread roots of this alternative embodiment also provides increased fatigue strength over drill pipe in which the threads are not cold worked.

Cold working of external threads may conveniently be performed by rotating the pipe section against a tool such as that indicated at H. Figure 1. Tool 1'! includes a roller I 8 having an edge [9 which fits within the pipe thread and contacts only the thread root. A hydraulic cylinder or other means, not shown, may be provided for applying suflicient pressure to the roller mounting for performing the cold work on the thread root. While the roller is illustrated as having but one root contacting edge, it is apparent that rolls having multiple edges likewise could be employed, and are preferred for cold working roots of multiple threads. Inasmuch as the tool and pipe rotating mechanism per se are not part of the present invention and a great variety of arrangements are obviously. possible, no more detailed showing is deemed necessary.

Tests have demonstrated that the fatigue strength of drill pipe having threads cut and cold worked according to the preferred embodiment of the persent invention is increased as much as 140% over that of pipes having conven tional threads. The fatigue strength of drill pipe having threads cold worked according to the alternative embodiment is increased over that have extent as in the preferred embodiment.

As an example, comparative fatigue tests were conducted on 5 inch outside diameter, 0.362'inch is possible to obtain considerable improvement in the fatigue strength thereby on fiat and sharp roots. Modification of the V-thread, including the buttress, and acme types, may be so treated with benefit. v

While we have shown and described only certain methods and means in which the present invention is involved, it is apparent that modifications may arise. Therefore, we do not wish to be limited by the disclosure set forth, but only by the scope of the appended claims.

We claim:

1. In a steel drill pipe section having external screw threads for engagement by an internally screw threaded coupling and having fade-out threads, the threads being of the V-type and having rounded roots, means for counteracting the loss of bending fatigue strength in the threaded region comprising cold worked portions in the roots of said coupling-engaging and fade-out threads, the amount of cold reduction progressively increasing from the end of the pipe section to the region of the last thread adapted to be engaged bythe coupling and progressively decreasing through the fade-out threads.

2. In a steel drill pipe section having a tapered externally screw threaded end for engagement by an internally screw threaded coupling and having fade-out threads, the angle of fade-out being substantially equal to the angle of taper, the threads being of the V-type and having rounded roots, means for counteracting the loss of bending fatigue strength in the threaded region comprising cold worked portions in the roots of said coupling engaging and fade-out threads, the amount of cold reduction progressively increasing from the end of the pipe section to the region of the last thread adapted to be engaged by the coupling and progressively decreasing through the fade-out threads, the thread crests and sides being relatively soft.

3. A method for increasing bending fatigue 5 6 gradually diminishing through the fade-out Number Name Date threads. 2,243,608 Schaurte May 27, 1941 JERRY J DUNN 2,261,056 Dunn Oct. 23, 1941 WHLIAM O, CLINEDINST'. 2,284,260 Castellanos May 26, 1942 OTHER REFERENCES Pages 96 through 98, Shot Peening, published in 1946 by American Wheelabrator 8; Equipment References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Corp., Mishawaka, Ind.

631,158 Echols Aug. 15, 1899 10 Arnold Effect of Screw Threads On Fatigue, 2,073,093 Brantly Mar. 9, 1937 pages 497-505, Mechanical Engineering, July 2,173,003 Place Sept. 12, 1939 1943, published by the American Society of Me- 2,188,629 Giesey et a1 June 30, 1940 chanical Engineers, New York, N.Y. 

